The electrolytic field respirometer is an apparatus for maintaining a constant oxygen atmosphere over a biological sample while measuring the carbon dioxide produced by the specimen. It allows the determination of carbon turnover values (carbon mineralization rates) in systems such as soils, decomposing forest litter, sewage sludge applied to land, and animal waste disposal systems. It may be used also to study respiration of microbial cultures, small animals, and tissue preparations. It is capable of indirectly measuring the rate of oxygen consumption in the above systems, as well as total oxygen consumption. In conjunction with a gas chromatograph, it is capable of being used for nitrogen fixation studies wherein the utilized or fixed nitrogen in a sample vessel is replaced by oxygen and results in an increased O.sub.2 /N.sub.2 ratio in the sample vessel.
Early methods of measuring the rate of decomposition of organic matter relied upon periodic determination of the amount of carbon dioxide evolved by the sample. As an example, a definite area of soil might be covered with a container of known volume for a specific period of time. The carbon dioxide produced within the container during the test period would then be measured. In laboratory studies, the usual basic procedure was to keep a sample of soil in a vessel which also contained a vial of carbon dioxide absorbent. The carbon dioxide absorbed could be measured after a set period of time.
Incubation procedures are not fully satisfactory for the study of organic matter decomposition because the gradual depletion of oxygen during the test period affects microbial activity. One answer proposed to this problem was the use of barium peroxide to renew the diminishing oxygen supply during respiration. Other researchers supplied pure oxygen at atmospheric pressure, the oxygen being drawn into the sample vessel or flasks as carbon dioxide was absorbed. Another technique used in the part was to keep the soil in a vessel and draw over its surface a slow stream of carbon dioxide-free air. The air was then passed through a carbon dioxide absorbent and the carbon dioxide determined at known intervals of time.
In all of these prior methods, the necessary measurement of carbon dioxide evolution could be carried out only at definite intervals of time, not continuously, and none of these methods gave a complete picture of organic matter decomposition. It is desirable in this type of test to know the quantity and rate of oxygen uptake during decomposition. This information, plus periodic determination of carbon dioxide evolved, permits the calculation of the "respiratory quotient" CO.sub.2 /O.sub.2, which gives an indication of the type of organic material decomposing.
More recent efforts have introduced the use of an electrolytic soil respirometer for measuring oxygen uptake, providing continuous replacement of oxygen by electrolysis as decomposition proceeds. In general, the sample is placed in a container with a sealed cover. A material is placed in the container to absorb carbon dioxide and a tube containing an electrode connects the sample container to a solution of electrolyte. Another electrode is immersed in the electrolyte. As carbon dioxide is absorbed by the sample, it creates a partial vacuum and the electrolyte is drawn up the tube until it makes contact with the electrode. Electrolysis of the water in the electrolyte results in delivery of oxygen to the container until the pressure is restored. At that time, the electrolyte sinks to its original level and electrolysis ceases. Hydrogen liberated during electrolysis can be measured at any time during or after cell operation.
Production of oxygen on demand by an electrolytic cell has been found unmanageable without external temperature controls, since the pressure fluctuations that result from temperature changes make the resulting reading unreliable. Prior electrolytic cell respirometers can be used only under laboratory conditions, where the sample container and electrolytic cell can be immersed in a controlled water bath or other incubator.
Laboratory use of the electrolytic respirometer in conjunction with testing of biological samples has demonstrated the versatility of this type of apparatus. The present apparatus was developed as a field unit, having internal temperature compensation and partial barometric compensation and eliminating the need for a water bath to control temperature. Basic requirements for the unit also were that it be small, simple, portable, versatile, and sufficiently inexpensive to be used in considerable numbers in field studies as well as in laboratory studies.